The invention relates to a conductor string extending from a marine platform into the bottom of a body of water.
When holes or wells are to be drilled in the seabottom by a drilling unit installed on a marine platform supported by the seabottom, the initial step in the drilling process includes the installation of a large diameter tube extending from a marine platform into the seabottom. This tube, hereinafter referred to as conductor string, consists of a plurality of pipe sections interconnected in end to end relationship and it may penetrate the seabottom to a certain depth. After the installation of the conductor string, a drill string carrying a drill bit at its lower end, is lowered through the conductor string for drilling a hole in the bottom. During the drilling operation, mud is supplied through the drill string to the bit for the purpose, amongst others, of removing cuttings from the hole which is being drilled. The mud leaving the bit is subsequently passed upwardly to the drilling unit through the annular space between the conductor string and the drill string. During the initial drilling period, the hydrostatic head of the mud column in the annular space is (in particular in deep water operations) considerably greater than the fracturing pressure of the formation layers of the seabottom in which the hole or well is being drilled. It will be appreciated that during the initial drilling period the hydrostatic head of the mud column should therefore be reduced to a value lower than the fracturing pressure of the formation, since fracturing of the formation may lead to loss of the hole being drilled or even to loss of the marine platform structure supporting the drilling unit. At a later stage of the drilling operation, when the hole has been drilled to a greater depth, at which depth the fracturing pressure is higher than the hydrostatic head of the mud column, a first casing is set and cemented in the hole, whereafter drilling is carried out in a normal manner.